1. Chapter 4 – Requirements Engineering
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Chapter 4 Requirements Engineering

2. Chapter 4 Requirements Engineering
Topics covered
Functional and non-functional requirements
Requirements engineering processes
Requirements elicitation
Requirements specification
Requirements validation
Requirements change
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3. Requirements engineering
Requirements engineering: the process of establishing the services that a customer requires from a system and the constraints under which it operates and is developed.
It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification.
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4. Chapter 4 Requirements Engineering
Types of requirement
User requirements
Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers.
System requirements
A structured document setting out detailed descriptions of the system’s functions, services and operational constraints. Defines what should be implemented so may be part of a contract between client and contractor.
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5. User and system requirements
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6. Chapter 4 Requirements Engineering
System stakeholders
Any person or organization who is affected by the system in some way and so who has a legitimate interest
Stakeholder types
End users
System managers
System owners
External stakeholders
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7. Stakeholders in the Mentcare system
Patients whose information is recorded in the system.
Doctors who are responsible for assessing and treating patients.
Nurses who coordinate the consultations with doctors and administer some treatments.
Medical receptionists who manage patients’ appointments.
IT staff who are responsible for installing and maintaining the system.
A medical ethics manager who must ensure that the system meets current ethical guidelines for patient care.
Health care managers who obtain management information from the system.
Medical records staff who are responsible for ensuring that system information can be maintained and preserved, and that record keeping procedures have been properly implemented.
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8. Agile methods and requirements
Many agile methods argue that producing detailed system requirements is a waste of time as requirements change so quickly.
This is practical for business systems but problematic for systems that require pre-delivery analysis (e.g. critical systems) or systems developed by several teams.
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9. Functional and non-functional requirements
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10. Functional and non-functional requirements
Statements of services the system should provide, how the system should react to particular inputs and how the system should behave in particular situations.
May state what the system should not do.
Non-functional requirements
Constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc.
Domain requirements
Constraints on the system from the domain of operation
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11. Functional requirements
Functional user requirements may be high-level statements of what the system should do.
Functional system requirements should describe the system services in detail.
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12. Mentcare system: functional requirements
A user shall be able to search the appointments lists for all clinics.
The system shall generate each day, for each clinic, a list of patients who are expected to attend appointments that day.
Each staff member using the system shall be uniquely identified by his or her 8-digit employee number.
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13. Requirements imprecision
Problems arise when functional requirements are not precisely stated.
Ambiguous requirements may be interpreted in different ways by developers and users.
Consider the term ‘search’ in requirement 1
User intention – search for a patient name across all appointments in all clinics;
Developer interpretation – search for a patient name in an individual clinic. User chooses clinic then search.
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14. Requirements completeness and consistency
In principle, requirements should be both complete and consistent.
Complete
They should include descriptions of all facilities required.
Consistent
There should be no conflicts or contradictions in the descriptions of the system facilities.
In practice, it is impossible to produce a complete and consistent requirements document.
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15. Non-functional requirements
These define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc.
Process requirements may also be specified mandating a particular IDE, programming language or development method.
Non-functional requirements may be more critical than functional requirements. If these are not met, the system may be useless.
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16. Types of nonfunctional requirement
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17. Non-functional requirements implementation
Non-functional requirements may affect the overall architecture of a system rather than the individual components.
For example, to ensure that performance requirements are met, you may have to organize the system to minimize communications between components.
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18. Non-functional classifications
Product requirements
Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc.
Organisational requirements
Requirements which are a consequence of organisational policies and procedures e.g. process standards used, implementation requirements, etc.
External requirements
Requirements which arise from factors which are external to the system and its development process e.g. legislative requirements, etc.
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19. Examples of nonfunctional requirements in the Mentcare system
Product requirement
The Mentcare system shall be available to all clinics during normal working hours (Mon–Fri, 0830–17.30). Downtime within normal working hours shall not exceed five seconds in any one day.
Organizational requirement Users of the Mentcare system shall authenticate themselves using their health authority identity card.
External requirement The system shall implement patient privacy provisions as set out in HStan priv.
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20. Goals and requirements
Non-functional requirements may be very difficult to state precisely and imprecise requirements may be difficult to verify.
Goal
A general intention of the user such as ease of use.
Verifiable non-functional requirement
A statement using some measure that can be objectively tested.
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21. Usability requirements
The system should be easy to use by medical staff and should be organized in such a way that user errors are minimized. (Goal)
Medical staff shall be able to use all the system functions after four hours of training. After this training, the average number of errors made by experienced users shall not exceed two per hour of system use. (Testable non-functional requirement)
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22. Metrics for specifying nonfunctional requirements
Property
Measure
Speed
Processed transactions/second
User/event response time
Screen refresh time
Size
Mbytes
Number of ROM chips
Ease of use
Training time
Number of help frames
Reliability
Mean time to failure
Probability of unavailability
Rate of failure occurrence
Availability
Robustness
Time to restart after failure
Percentage of events causing failure
Probability of data corruption on failure
Portability
Percentage of target dependent statements
Number of target systems
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23. Requirements engineering processes
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24. Requirements engineering processes
The processes used for RE vary widely depending on the application domain, the people involved and the organisation developing the requirements.
However, there are a number of generic activities common to all processes
Requirements elicitation;
Requirements analysis;
Requirements validation;
Requirements management.
In practice, RE is an iterative activity in which these processes are interleaved.
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25. A spiral view of the requirements engineering process
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26. Requirements elicitation
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27. Requirements elicitation and analysis
Sometimes called requirements elicitation or requirements discovery.
Involves technical staff working with customers to find out about the application domain, the services that the system should provide and the system’s operational constraints.
May involve end-users, managers, engineers involved in maintenance, domain experts, trade unions, etc. These are called stakeholders.
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28. Requirements elicitation
Stages include:
Requirements discovery,
Requirements classification and organization,
Requirements prioritization and negotiation,
Requirements specification.
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29. Chapter 4 Requirements Engineering
Process activities
Requirements discovery
Interacting with stakeholders to discover their requirements. Domain requirements are also discovered at this stage.
Requirements classification and organisation
Groups related requirements and organises them into coherent clusters.
Prioritisation and negotiation
Prioritising requirements and resolving requirements conflicts.
Requirements specification
Requirements are documented and input into the next round of the spiral.
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30. Chapter 4 Requirements Engineering
Interviewing
Types of interview
Closed interviews based on pre-determined list of questions
Open interviews where various issues are explored with stakeholders.
Effective interviewing
Be open-minded, avoid pre-conceived ideas about the requirements and are willing to listen to stakeholders.
Prompt the interviewee to get discussions going using a springboard question, a requirements proposal, or by working together on a prototype system.
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31. Problems of requirements elicitation
Stakeholders don’t know what they really want.
Stakeholders express requirements in their own terms.
Different stakeholders may have conflicting requirements.
Organisational and political factors may influence the system requirements.
The requirements change during the analysis process. New stakeholders may emerge and the business environment may change.
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32. Requirements specification
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33. Requirements specification
The process of writing down the user and system requirements in a requirements document.
User requirements have to be understandable by end- users and customers who do not have a technical background.
System requirements are more detailed requirements and may include more technical information.
The requirements may be part of a contract for the system development
It is therefore important that these are as complete as possible.
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34. Ways of writing a system requirements specification
Notation
Description
Natural language
The requirements are written using numbered sentences in natural language. Each sentence should express one requirement.
Structured natural language
The requirements are written in natural language on a standard form or template. Each field provides information about an aspect of the requirement.
Design description languages
This approach uses a language like a programming language, but with more abstract features to specify the requirements by defining an operational model of the system. This approach is now rarely used although it can be useful for interface specifications.
Graphical notations
Graphical models, supplemented by text annotations, are used to define the functional requirements for the system; UML use case and sequence diagrams are commonly used.
Mathematical specifications
These notations are based on mathematical concepts such as finite-state machines or sets. Although these unambiguous specifications can reduce the ambiguity in a requirements document, most customers don’t understand a formal specification. They cannot check that it represents what they want and are reluctant to accept it as a system contract
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35. Example requirements for the insulin pump software system
3.2 The system shall measure the blood sugar and deliver insulin, if required, every 10 minutes. (Changes in blood sugar are relatively slow so more frequent measurement is unnecessary; less frequent measurement could lead to unnecessarily high sugar levels.)
3.6 The system shall run a self-test routine every minute with the conditions to be tested and the associated actions defined in Table 1. (A self-test routine can discover hardware and software problems and alert the user to the fact the normal operation may be impossible.)
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36. Form-based specifications
Definition of the function or entity.
Description of inputs and where they come from.
Description of outputs and where they go to.
Information about the information needed for the computation and other entities used.
Description of the action to be taken.
Pre and post conditions (if appropriate).
The side effects (if any) of the function.
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37. A structured specification of a requirement for an insulin pump
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38. A structured specification of a requirement for an insulin pump
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39. Chapter 4 Requirements Engineering
Use cases
Use-cases are a kind of scenario that are included in the UML.
Use cases identify the actors in an interaction and which describe the interaction itself.
A set of use cases should describe all possible interactions with the system.
High-level graphical model supplemented by more detailed tabular description (see Chapter 5).
UML sequence diagrams may be used to add detail to use-cases by showing the sequence of event processing in the system.
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40. Use cases for the Mentcare system
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41. The software requirements document
The software requirements document is the official statement of what is required of the system developers.
Should include both a definition of user requirements and a specification of the system requirements.
It is NOT a design document. As far as possible, it should set of WHAT the system should do rather than HOW it should do it.
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42. Users of a requirements document
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43. Requirements validation
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44. Requirements validation
Concerned with demonstrating that the requirements define the system that the customer really wants.
Requirements error costs are high so validation is very important
Fixing a requirements error after delivery may cost up to times the cost of fixing an implementation error.
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45. Requirements checking
Validity. Does the system provide the functions which best support the customer’s needs?
Consistency. Are there any requirements conflicts?
Completeness. Are all functions required by the customer included?
Realism. Can the requirements be implemented given available budget and technology
Verifiability. Can the requirements be checked?
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46. Requirements validation techniques
Requirements reviews
Systematic manual analysis of the requirements.
Prototyping
Using an executable model of the system to check requirements. Covered in Chapter 2.
Test-case generation
Developing tests for requirements to check testability.
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47. Chapter 4 Requirements Engineering
Review checks
Verifiability
Is the requirement realistically testable?
Comprehensibility
Is the requirement properly understood?
Traceability
Is the origin of the requirement clearly stated?
Adaptability
Can the requirement be changed without a large impact on other requirements?
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48. Requirements management
Requirements management is the process of managing changing requirements during the requirements engineering process and system development.
New requirements emerge as a system is being developed and after it has gone into use.
You need to keep track of individual requirements and maintain links between dependent requirements so that you can assess the impact of requirements changes. You need to establish a formal process for making change proposals and linking these to system requirements.
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49. Requirements change management
Deciding if a requirements change should be accepted
Problem analysis and change specification
During this stage, the problem or the change proposal is analyzed to check that it is valid. This analysis is fed back to the change requestor who may respond with a more specific requirements change proposal, or decide to withdraw the request.
Change analysis and costing
The effect of the proposed change is assessed using traceability information and general knowledge of the system requirements. Once this analysis is completed, a decision is made whether or not to proceed with the requirements change.
Change implementation
The requirements document and, where necessary, the system design and implementation, are modified. Ideally, the document should be organized so that changes can be easily implemented.
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50. Requirements change management
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